function search_word_replace($buffer){
    if(is_search()){
        $arr = explode(" ", get_search_query());
        $arr = array_unique($arr);
        foreach($arr as $v)
            if($v)
                $buffer = preg_replace("/(".$v.")/i", "$1", $buffer);
    }
    return $buffer;
}
add_filter("the_title", "search_word_replace", 200);
add_filter("the_excerpt", "search_word_replace", 200);
add_filter("the_content", "search_word_replace", 200);

加载主题文件夹下的functions.php中，嵌套em标签，可以根据需要调整标签样式。Via: http://www.cmhello.com/wordpress-search-keyword-highlight.html